The invention relates to a system and method for controlling a solar collector or solar concentrator. More specifically, the system and method relate to a controller system and associated software for accepting sensor inputs from sensors on the solar collector to determine the state of operation of an energy conversion device associated with the concentrator. In particular, the sensors provide information relative to the energy conversion device operating above a certain capacity, indicated, for example, by overheating, and the controller system issues commands to control the focusing of the solar collector, and executing instructions based on the energy conversion device condition to avoid operation above a desired capacity, while maintaining the energy conversion device online and operational.
Solar collector or concentrator systems are used to collect solar energy from sunlight and convert it to a usable form of energy. The terms xe2x80x9csolar collector,xe2x80x9d xe2x80x9csolar concentrator,xe2x80x9d xe2x80x9ccollector,xe2x80x9d xe2x80x9cconcentrator,xe2x80x9d and xe2x80x9csolar dishxe2x80x9d or xe2x80x9cdishxe2x80x9d are used interchangeably herein to indicate the collector and concentrator portion of the solar collector, although, as would be understood by one of ordinary skill in the art, a solar collector or concentrator is not necessarily dish-like in shape.
One example of converting solar energy to usable energy is that solar energy may be stored in a battery for future use, or it may be used to generate power using a solid state device or an engine system. Such devices are referred to herein as a Power Conversion System (xe2x80x9cPCSxe2x80x9d). One such engine system commonly used in solar collector systems is a Stirling engine, which is a type of engine that derives mechanical power from the expansion of a confined gas at a high temperature. However, the system and method disclosed herein may be adapted for use with any PCS.
For example, other types of PCS""s may include photovoltaic cells which convert light energy into electricity. For purposes of this description, all such types of PCS systems and devices are generally referred to herein as xe2x80x9cenergy conversion device(s).xe2x80x9d
Solar collector systems typically include motion controlling systems to change the orientation of the collector. As the sun moves across the sky, the solar collector orientation must be changed accordingly to track the position of the sun by compensating for the earth""s rotation. One complication arising from the use of solar collectors or concentrators is that high wind conditions may cause damage to solar collector systems because solar collectors are typically placed on a pedestal above the ground. Therefore, to avoid such damage, the solar collector is normally lowered or stowed in a safer orientation if high wind conditions exist.
The motors and drive systems used to control the orientation of a solar collector system may be controlled electronically by some combination of manual commands entered by a user. Alternatively, sensors may be placed to monitor various conditions of the solar collector, and a microprocessor may issue commands to change the orientation of the solar collector system based on the sensor inputs.
Current programming techniques used on such microprocessors are based on a hierarchical methodology. As used herein, the terms xe2x80x9cprogram algorithm,xe2x80x9d xe2x80x9cprogram routine,xe2x80x9d xe2x80x9cprogram subroutine,xe2x80x9d xe2x80x9calgorithm,xe2x80x9d xe2x80x9croutine,xe2x80x9d and xe2x80x9csubroutinexe2x80x9d are used interchangeably to refer to any block of code that may be logically grouped together and may or may not use the conventional subroutine interfaces as defined by typical programming languages. As would be understood by one of ordinary skill in the art, a program routine or subroutine is generally understood as a stylistic convention of programming, and thus different routines or subroutines may be written in multiple combinations and accomplish the same function. Thus, as used herein, a program algorithm, routine or subroutine encompasses any block of code logically grouped together regardless of whether conventional subroutine interfaces, as defined by typical programming languages, are used.
In a hierarchical program, the programming algorithm operates in a sequential manner, and the orientation of the solar collector is known to a system operating in accordance with the algorithm, based on previously issued commands. For example, the programming algorithm is initialized to certain starting parameters to indicate the starting orientation of the solar collector. If a user enters a command to place the solar collector into an operational state, the system implementing the programming algorithm issues instructions to the motors and drive systems to move a given direction in order to be placed in operational orientation. If the solar collector is moved again, for example, to track the sun, the information from the previously executed commands is used to determine what commands must be issued to re-orient the solar collector. By xe2x80x9cstatexe2x80x9d or xe2x80x9ccollector statexe2x80x9d is meant the combination of all the known status indicators of the collector, which may include positional orientation, temperature, wind conditions, etc.
If an error in the system occurs, it is difficult or impossible to issue new commands correctly. That is, if the program implementing the algorithm is unable to determine the correct orientation of the solar collector from its past history, it cannot accurately issue new commands or instructions. Error detection is also difficult in such a system. If the program implementing the algorithm has an error, it will continue to operate even though it may be issuing commands based on incorrect assumptions about the solar collector orientation. If such a system is turned off and restarted in mid-operation, the program routine does not have correct starting parameters, and therefore, is unable to issue correct control commands.
One particular type of solar collector system currently in use involves the use of a concentrator having stretched membrane mirror facets. Such systems have been installed and are known commercially by the name SunDish(trademark). Such systems have has been operationally installed through the cooperation of The Salt River Project (SRP), Science Applications International Corporation (SAIC), STM Corporation and the U.S. Department of Energy. Further details about such a system are disclosed in a document entitled The Salt River Project SunDish(trademark) dish-Stirling System, authored by Jessica Mayette (Salt River Project), Roger L. Davenport and Russell Forristall (SAIC).
Such a concentrator typically has 16 round, stretched membrane mirror facets. The stretched membrane mirror facets consist of a rolled steel ring with stainless steel membranes welded to the front and back surfaces of the ring.
Thin, typically 1-mm, low-iron glass mirrors, attached with adhesive to the front membrane, provide the reflective surface. The facets are focused by pulling a slight vacuum between the membranes using a blower system. Such a system allows fine-tune adjustment of the focal length during alignment of the system.
The use of such membranes, however, may give rise to complications in operation. More specifically, in the case of dish-engine systems where an energy conversion device such a Stirling engine is used, it is normally desirable to operate the engine near its peak power point to optimize efficiency. However, solar energy varies seasonally and over the course of the day. If the engine cannot accept the power available from the concentrator at any given time, due to optional focusing at high solar radiation levels, it will overheat.
One solution has been to off-track, i.e., no longer track the sun, with the dish, but this drops system output to zero, and may overheat other components that the beam tracks across. The other solution is to oversize the engine relative to the dish so that it never sees more power from the dish than it can handle. This is costly, and leads to lower system efficiency since the engine operates lower on its power curve most of the time.
Such overheating can also occur in the case where a concentrator includes fixed focus mirrors and the power input to an energy conversion device exceeds a predetermined amount. As noted, such energy conversion devices can also include photovoltaic devices, as contrasted with Stirling engines, where voltage output monitored exceeding a predetermined value, i.e., overpowering, may indicate an overheating condition.
In accordance with the system and method described herein, these and other problems are avoided by providing a disk system which allows for operation of an engine conversion device near its peak, or other desired level, and controls or avoids overheating while continuing to maintain the dish system operational with power output continuing from the energy conversion device.
In one aspect the invention relates to a system for controlling a solar concentrator. The system includes a concentrator having multiple mirrors and an energy conversion device, such as a Stirling engine or photovoltaic array, associated with the solar concentrator for having sunlight reflected from the multiple mirrors focused thereon. An arrangement is provided for focusing and defocusing the multiple mirrors of the solar concentrator on the energy conversion device. A sensor serves to monitor the operation of the energy conversion device to provide an output indicative of whether the amount of solar energy focused on the device is at a maximum amount or other predetermined level at which the energy conversion device is to operate. A controller system serves to control the operation of the solar concentrator, the multiple mirrors, and the energy conversion device. The controller system is configured for comparing the output of the sensor with a first predetermined value below the value indicative of the solar energy focused on the energy causing operation of the energy conversion device at a maximum or other predetermined level at which it is to operate, and serves to either focus or defocus at least one of the multiple mirrors to cause the output of the sensor to fall below the first predetermined value. The controller system is further configured for comparing the output of the sensor with a second predetermined value below the first predetermined value for refocusing the multiple mirrors on the energy conversion device once the second predetermined value is reached.
In a more specific aspect, the multiple mirrors are flexible membrane mirrors, and the system includes a blower for causing the flexible membrane mirrors to focus incident sunlight on the energy conversion device when the blower is in operation, typically, by pulling as light vacuum on the membrane mirrors.
In a yet still more specific aspect, the energy conversion device is a Stirling engine, the sensor is a temperature sensor, and the blower makes up the arrangement for focusing and defocusing, including a blower controller for causing the blower to turn on and off in response to signals from the controller system generated in response to the output received from the temperature sensor.
In an alternative arrangement, the multiple mirrors are fixed focus mirrors mounted on the solar concentrator, in which they can be moved to a position in which reflected solar energy is not focused on the energy conversion device. The arrangement for focusing and defocusing may include moving means, for example, individual motors associated with each of the individual mirrors which are mounted for moving or pivoting, for moving at least one of the multiple mirrors between at least two positions, a first position in which the mirror focuses reflected solar radiation on the energy conversion device, and a second position in which the reflected solar radiation is directed away from the energy conversion device.
In both arrangements, as noted previously, such an energy conversion device can either be a Stirling engine, a photovoltaic device, or other type energy conversion device or system. For all types of energy conversion devices, while a temperature sensor has been indicated as one way of monitoring its condition, other arrangements, for example, such as monitoring the amount of electricity generated by a photovoltaic device as indicative of its operational condition can be employed in an alternative construction.
In an alternative, there is disclosed a method of controlling a solar concentrator system which includes a solar concentrator with at least one mirror for focusing reflected sunlight on an energy conversion device. The solar concentrator system includes an energy conversion device associated with the solar concentrator. The method includes providing a means for focusing and defocusing reflected sunlight from the at least one mirror on the energy conversion device. Such means can include a blower assembly in association with flexible membrane mirrors by which, depending on whether the blower assembly is turned on or off, the mirrors are either focused or defocused relative to incident sunlight reflected to the energy conversion device. The level of operation of the energy conversion device is monitored, with the energy conversion device having a predetermined maximum level of operation at which the operation of the energy conversion device is shut down. It is determined that the level of operation has reached a first predetermined level of operation below the maximum level, and if the first predetermined level of operation has been reached, the at least one mirror is defocused to reduce the intensity of reflected light directed onto the energy conversion device while maintaining the energy conversion device in operation. The level of operation of the energy conversion device is further monitored to determine if it reaches a second predetermined level lower than the first predetermined level, and when the second predetermined level is reached, the at least one mirror is refocused.
In a yet more specific aspect, the energy conversion device is a Stirling engine and the monitoring involves monitoring the temperature of the Stirling engine. Yet more specifically, the concentrator system includes a plurality of flexible membrane mirrors and a blower for focusing the mirrors when in operation, wherein the mirrors are focused and defocused by turning the blower on and off.